Integrated circuits (ICs) are typically fabricated with multiple levels of patterned metallization lines, electrically separated from one another by interlayer dielectrics containing vias at selected locations to provide electrical connections between levels of the patterned metallization lines. As these integrated circuits are scaled to smaller dimensions in a continual effort to provide increased density and performance (e.g., by increasing device speed and providing greater circuit functionality within a given area chip), the interconnect line width dimension becomes increasingly narrow, which in turn renders them more susceptible to deleterious effects such as electromigration.
Electromigration is a term referring to the phenomenon of mass transport of metallic atoms (e.g., copper or aluminum) which make up the interconnect material, as a result of unidirectional or DC electrical current conduction. More specifically, the electron current collides with the diffusing metal atoms, thereby pushing them in the direction of current travel. Over an extended period of time, the accumulation of metal at the anode end of the interconnect material significantly increases the local mechanical stress in the system. This in turn may lead to delamination, cracking, and even metal extrusion from the metal wire, thereby causing an electrical short to adjacent interconnects. Electromigration becomes increasingly more significant in integrated circuit design, as relative current densities through metallization lines continue to increase as the line width dimensions shrink.
In addition to electromigration, other factors such as time dependent dielectric breakdown (TDDB) are also factors in semiconductor reliability. As critical dimensions (CD) continue to shrink, the spacing between two metal lines reduces as well. Moreover, as the CD shrinks, it also provides challenges for liner deposition. Poor liner coverage will lead to the diffusion of the liner material, for example copper, to diffuse into the dielectric layer adjacent to it. The destruction of the dielectric layer can cause interconnects to short, rendering the IC defective. Unfortunately, steps taken to mitigate electromigration can adversely affect TDDB. It is therefore desirable to have a structure and method for improving yield and reducing defects caused by both electromigration and TDDB.